Ultrasonic transducers have been employed in ultrasound therapy systems to achieve therapeutic heating of diseased and other tissues. Arrays of ultrasound transducers operating to form a beam of ultrasonic energy cause a conversion of sound to thermal energy in the affected tissue areas or treatment volumes, and a subsequent beneficial rise in the temperature in the treatment volumes.
In image-guided ultrasound therapy systems, a patient and the ultrasound therapy apparatus are generally disposed in an imaging volume such as a magnetic resonance imaging (MRI) apparatus, which allows guidance of the applicator placement, and in addition allows monitoring of the treatment effect on the tissue by providing real-time data from which temperature maps can be calculated. A clinical operator can then monitor the progress of the therapy within the treatment volume or diseased tissue and manual or automated changes can be made to the ultrasound power signals based on input from the results and progress of the treatment. With proper monitoring of the heating effect, ultrasound therapy systems can be used to treat harmful cells and to controllably destroy tumors.
The temperature created by the absorption of sound in a sound-conducting medium is not uniform. When the acoustic field is not generally focused, the temperature rise is highest close to the source of sound and it decreases with distance from the source. The sound created by a piston-shaped transducer is highly directional. As such there will be an increase in temperature along the line perpendicular to the center of the face of the piston with only small increases in temperature in the volumes adjacent to that perpendicular line. The resultant shape of thermal energy deposition is similar to the flame from a match with a narrow tip and being slightly wider at the base.
In any material, local temperature differences gradually disappear due to heat transfer from areas of high temperature to areas of lower temperature. In live tissue thermal diffusion and blood circulation are two of the main mechanisms by which heat transfer take place. If there is an area of increased temperature in tissue, these heat transfer phenomena work to reduce the peak temperature and increase the surrounding tissue temperature.
Work has been done to demonstrate the use of magnetic resonance imaging (MRI) guided transurethral ultrasound therapy systems for treatment of disease such as prostate cancer in men. See, e.g., Chopra, et al., “MRI-compatible transurethral ultrasound system for the treatment of localized prostate cancer using rotational control,” Med Phys 35(4):1346-1357, 2008. Also see, U.S. Pub. 2007/0239062; U.S. Pat. No. 6,589,174 “Technique and apparatus for ultrasound therapy,” 2003; U.S. Pat. No. 7,771,418 “Treatment of diseased tissue using controlled ultrasonic heating,” 2010. Such systems, including cumulative published and patented work by or for the present applicant, all of which are hereby incorporated by reference, teach the use of transurethral ultrasonic energy to the diseased prostate to reach a desired target temperature in the diseased tissue to achieve the clinical result, which is usually the necrosis of the diseased tissue cells in the prostate. MRI guidance and temperature monitoring of the treatment in realtime enables control of the power to the ultrasound therapy transducers as well as control of the rotation of an array of such transducers disposed axially along an elongated applicator inserted into the patient's urethra in the vicinity of the diseased prostate.
It is understood that it is necessary to control the operation of such systems in use, as uncontrolled, or poorly controlled, operations can lead to unwanted injury to the patient through overheating the patient's tissue or applying the heat treatment to organs and tissues that should not be treated. See, e.g., U.S. Pub. 2007/0239062 “Method and apparatus for obtaining quantitative temperature measurements in prostate and other tissue undergoing thermal therapy treatment,” 2007; U.S. Pub. 2006/0206105 “Treatment of diseased tissue using controlled ultrasonic heating,” 2006.
One concern relates to the obvious harm of unwanted cell death from overheating healthy or critical organ tissue in the context of prostate treatment. Another concern relates to acoustic factors that can degrade or impede the operation of the therapy system if tissue proximal to the therapy system operated in a way that causes boiling (approximately 100 Celsius) or cavitation (formation of gas voids in the tissue) in the tissue. These effects may be beneficial or desired in some contexts, addressed elsewhere, but for the present purpose, unless stated otherwise, the preferred embodiments below rely on temperature control rather than mechanical, boiling, cavitation or other effects to achieve their desired result. These concerns are recognized but not suitably or perfectly solved for all situations in the presently-cited and similar references in the field.
Still other work has been published describing the real and simulated effects of ultrasound thermal therapy systems. See, e.g., Burtnyk et al., “Quantitative analysis of 3-D conformal MRI-guided transurethral ultrasound therapy of the prostate: theoretical simulations,” Int J Hyperthermia 25(2): 116-131, 2009; Burtnyk et al., “Simulation study on the heating of the surrounding anatomy during transurethral prostate therapy: A 3-D theoretical analysis of patient safety,” Med Phys 37(6): 2862-2875, 2010. Again, the above and similar efforts indicate a recognition of the need to control, measure, predict and otherwise understand the effects of conformal thermal therapy systems.
Yet another aspect of conformal thermal therapy treatment is that of time-dependence and the three-dimensional nature of heat conduction and diffusion. If a thermal treatment leads to a certain temperature next to a target boundary in the treatment zone, it is possible for the target temperature at the target boundary to be exceeded by heat transfer from an adjacent area with higher temperature.
The present disclosure and inventions address, among other aspects, the above issues and cover systems and methods for better thermal treatment in patients suffering from disease such as prostate cancer.